A bug issue type is mapped to Case Mapping
An epic issue type is mapped to a CustomObject Mapping
Steps to replicate the limitation:
Associate a JIRA bug issue to Case record (let's call this Case1).
The mapping has JIRA issue-type mapped to a Salesforce custom-field (which is synched).
The SF custom-field has the value of Bug
Move the JIRA issue to Epic
After completing the move from Bug to Epic, the original Case1 will still have the custom-field as Bug (not Epic).
This is expected because Epic issue-type has the mapping to a CustomObject, so the the original Case1 is not updated
This idea is to have the ability to push/sync to Salesforce for an object that was previously mapped (Case1)